For a structural member used for an automobile, from the viewpoint of improving safety and the viewpoint of vehicle body weight reduction for fuel consumption improvement as the measures for environment problems, high strengthening has been required. For such a structural member, improvement of corrosion resistance has also been required.
As a raw material having both of the strength and corrosion resistance, a hot-dip galvannealed steel sheet (may be hereinafter referred to as a GA steel sheet) is used which is obtained by subjecting the surface of a base steel sheet to hot-dip galvanizing and alloying the same. In order to exert the corrosion resistance, it is required for the GA steel sheet that there is not a non-coated portion, the surface appearance is excellent, and the hot-dip galvannealed layer does not peel off from the base steel sheet (may be hereinafter referred to as coated-layer adhesiveness).
As a technology for improving the adhesiveness of the interface between the hot-dip galvannealed layer and the base steel sheet of the GA steel sheet, Patent Literature 1 can be cited for example. In Patent Literature 1, it is described that the coated-layer adhesiveness can be improved by making the interface between the coated layer and the base steel sheet after alloying treatment is made a complex state in which unevenness is high and the coated layer and the base steel sheet are complicatedly arranged. More specifically, it is described to be effective to contain Si of a predetermined amount and to achieve a state of high surface roughness of 6.5 μm or more in terms of 10 point mean roughness Rz of the steel surface surface roughness after removing the hot-dip galvannealed layer.
Also, in Patent Literature 2, the present inventors have disclosed a technology for improving the slidability and powdering resistance of the GA steel sheet with the aim of improving the workability of the GA steel sheet. According to the technology, the slidability and powdering resistance of the GA steel sheet have been improved by properly controlling the containing balance of Mn, P, Cr, Mo out of the chemical composition of the high strength steel sheet.
On the other hand, the shape of the structural members described above is becoming complicated year by year, and the GA steel sheet may possibly be subjected to working accompanied by sliding. Therefore, provision of a GA steel sheet whose hot-dip galvannealed layer hardly peels off from the base steel sheet in sliding working has been desired.